The present invention relates to energy storage systems for a vehicle and in particular to a battery system for a hybrid vehicle.
Hybrid electric vehicles employ an internal combustion engine and an electric motor which can alternatively, or in conjunction, provide a driving force for a vehicle.
There are several types of electric propulsion systems for vehicles. For example, a pure electric drive vehicle, a series hybrid system, a parallel hybrid system, and a combined series-parallel hybrid system are just a few of the designs currently being considered.
One common factor of all these designs is that there is a need for a high-efficiency, battery storage system.
Since many of the functions of the hybrid electric vehicle involve charging the batteries and then using this energy at a later time, the performance of the hybrid system is highly dependant on the performance of the battery. The choice of a battery to meet all requirements is critical. As with any design, many factors are involved in choosing the best technology to meet the requirements. Some of these factors are: performance, cost, weight, volume, and thermal characteristics, etc.
Some hybrid electric vehicle configurations replace the standard starting and generating systems of the vehicle with higher power, higher performance hybrid components. Therefore, finding an energy source system that meets the hybrid vehicle""s rigorous requirements is one the greatest challenges facing this technology.
Some of the factors that are associated with the energy storage system requirements are: power capability, energy capacity, life, cost, volume, mass, and temperature characteristics etc.
Nickel metal hydride (NiMH) batteries have become an attractive technology for many automobile manufacturers. NiMH batteries have high power capabilities, long life, lightweight, and have high packaging efficiency. However, NiMH batteries also have some drawbacks when compared to other battery technologies. NiMH batteries are expensive and have decreased performance at lower temperatures.
Since it is a requirement that the battery system provide enough power to start an engine at low temperatures, it is desirable to have a battery system which includes NiMH batteries while also accommodating the shortfalls of NiMH battery output at low temperatures.
The proposed solution to the NiMH cold temperature problem is to supplement the NiMH battery system with a secondary battery system. The preferred supplemental battery system is a battery having high cold cranking power. One example of a proposed supplemental battery is a Lead Acid (PbA) battery having spiral wound thin film technology.
When these two battery technologies are connected together in parallel, they complement each other in their respective weak areas of operation.
At cold temperatures, the NiMH""s internal resistance increases greatly while the PbA does not increase significantly. The resulting effect is that the PbA battery will stabilize the voltage of the battery system and supply the required current to start the engine. After the current transient, current is now traded from the NiMH battery to the PbA battery because of the disparity between the internal voltage potentials of each battery. This in effect will not allow the PbA battery to experience deep discharge, and therefore will increase its operating life.
At warmer temperatures, the NiMH battery will stabilize the voltage of the battery system during regenerative braking and absorb the majority of the regenerative braking energy. This will protect the PbA battery, as it will not allow the voltage to rise to a point where the PbA battery is in danger of damage.
During deeper discharge cycles, the NiMH battery voltage remains relatively constant vs. State of Charge (SOC). This allows the energy to be drawn from the NiMH battery while leaving the PbA battery at a high SOC.
In conclusion, the PbA battery helps the NiMH battery meet the cold performance requirements, and the NiMH keeps the PbA battery at a consistently high SOC to increase its life.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.